For I have known them all already, known them all:— Have known the evenings, mornings, afternoons, I have measured out my life with coffee spoons;

Meaning

Prufrock has over-planned and over-thought his existence. He has taken no chances. He’s led a “measured,” tame, domestic life. (Luckily he’s still alive, so he has time to change; but he feels this time slipping away.)

One often took coffee while making a social call in the 1900’s. The line suggests the amount of socializing Prufrock has done in his life: he’s so often calling on so-and-so that you could say he’s kept track of his life by the coffee he’s spooned out during visits. Also, coffee spoons are the smallest spoons in a traditional silver service, holding about ¼ teaspoon; Prufrock has doled out his life in tiny, prudent, decorous doses.

Since coffee, socializing, and wasting time have remained popular into the present day, this line has become one of the most famous in the poem. It’s even echoed in the song “Seasons of Love,” from the musical Rent:

How do you measure, measure a year?
In daylights, in sunsets,
In midnights, in cups of coffee…

Likewise, the Crash Test Dummies song “Afternoons and Coffeespoons,” a clear allusion to “Prufrock,” takes Eliot’s ideas and brings them into the 90s. This song adds to the discussion a fear of growing sick as well as old, and living your days out in the sterile, lifeless hallways of a hospital, measuring out the passing days with cups of coffee….